Recently, laser diodes utilizing compound semiconductor materials such as GaAs, etc. have been developed as optical devices for transmitting large-volume information at a high speed. Such optical devices comprise multi-layer thin films having hetero structures, and the thin films are generally formed by a gas-phase epitaxial method or a molecular beam epitaxial method. Because the optical devices are strongly required to have such characteristics as stability and long life, epitaxial layers should have as little defect density as possible. Accordingly, it is required that semiconductor substrates on which epitaxial layers are formed have a low dislocation density.
Substrates for devices are generally cut out from semiconductor single crystals. Methods for producing semiconductor single crystals include a gas phase growing method, a liquid phase growing method, and a solid phase growing method, and single crystals of compound semiconductors are mostly produced by the liquid phase growing method. Methods for growing single crystals from seed crystals by solidifying starting material melts, as one type of the liquid phase growth method, include a horizontal Bridgman method, a vertical Bridgman method, a gradient freezing method (GF method), and a Czochralski method (CZ method) and its improved method such as a liquid-encapsulated Czochralski method (LEC method), etc.
Recently, much attention is paid to a vertical Bridgman method (VB method) as a method for producing single crystals of compound semiconductors having large diameters of 3 inches (76.2 mm) or more and a low dislocation density, to achieve the mass-production of optical devices. For instance, when a single crystal of a GaAs compound semiconductor is formed by the VB method, starting materials consisting of Ga and As or GaAs are charged into a crystal-growing container having a bottom on which a GaAs seed crystal is disposed, the crystal-growing container containing a starting material melt obtained by heating is moved in a space having a temperature gradient in a vertical direction, so that crystallization occurs from the lower side (from the side of the seed crystal) toward the upper side. As a result, a single crystal grows from the seed crystal in a direction in perpendicular to its surface. Thus, the VB method can form high-quality, large-diameter single crystals of compound semiconductors with few crystal defects.
Because starting materials for a compound semiconductor single crystal usually include high-vapor pressure elements, a sealable crystal-growing container is used to produce the compound semiconductor single crystal. Because crystal growth is carried out at extremely small temperature gradients both in a growth orientation of the single crystal and in a radial direction thereof in perpendicular to the growth orientation, the generation of thermal strain is suppressed to obtain a low-dislocation single crystal.
In the vertical GF method, with a container containing a single crystal at its lower end located at a fixed position, the temperature is lowered while maintaining a vertical temperature gradient to subject the starting materials in the container to a similar thermal hysteresis to that in the vertical Bridgman method, to cause solidification by cooling, so that a single crystal is caused to grow in a direction in perpendicular to the surface of the seed crystal. Except for these points, it is essentially the same as the VB method.
However, because the surface orientations of device-forming surfaces of single crystal substrates have conventionally been mostly <100>, the crystal growth axis directions of the seed crystals used for growing the single crystals have also been generally <100>. However, depending on the types of epitaxial methods and device structures (thin film structures), the device-forming surface of the single crystal substrate is not necessarily a (100) face, but the surface orientations offsetting from the (100) face by an angle θ of about 2°, 10° or 15° have recently become used widely. Therefore, offset substrates were conventionally cut out in a direction inclined by the predetermined angle θ from the single crystal growing in a <100> direction. Here, the crystal orientation of <100>+θ is called “offset orientation,” and the slanting angle θ is called “offset angle.”
Because the crystal growth surface is horizontal in a crystal growth method such as a vertical Bridgman method, the concentration of a dopant (Si, Zn, etc.) is uniform in a radial direction of the single crystal. However, because the dopant, etc. tend to be segregated in the growth orientation, there is likely a concentration gradient of the dopant, etc. in a growth orientation in the grown single crystal. However, because in-plane uniformity, particularly the uniformity of a carrier concentration, is important for substrates used for optical devices, etc., the non-uniformity of a carrier concentration leads to unevenness in the characteristics of devices formed on the substrate surface. Accordingly, the substrates are required to have high uniformity in the carrier concentration in their surfaces.
On the other hand, in the case of a compound semiconductor single crystal obtained by crystal growth, for instance, from a seed crystal whose crystal growth axis direction is <100> according to the conventional method, a surface orientation-slanted substrate obtained by slantingly slicing the single crystal inherits the above dopant concentration distribution in the single crystal growth axis direction, whereby the dopant concentration distribution (thus carrier concentration distribution) in the substrate surface is disadvantageously non-uniform.
In addition, in the conventional method for cutting a surface orientation-slanted substrate slantingly from a growth orientation, there are large regions in both vertical end portions of the single crystal, from which substrates of the desired size cannot be cut out, resulting in low efficiency (yield) in utilizing the single crystal.